1994-2002 [Subscribe to Daily Digest] |
No problem. The ECU monitors the signal from the absolute pressure sensor, and when the pressure exceeds the range of the ECU's mapping, or an arbitrary limit, the ECU stops opening the fuel injectors. This fuel cut pressure is a bit over +1 bar.
The ECU is a powerful 32 bit computer and will not be doing simple map lookups. It will probably be parametric in operation and look up parametric parameters for difference operating conditions. So I can see how the boost fuel cut pressure point could be moved with little other adjustment being required. A couple of those with Abbott ECUs are running with MBC's. The Abbott fuel cut pressure is reported to be up around 22 PSI.
The ECU knows how to map fuel and provide boost enrichment up to this point, so everything works well.
The MBC is not involved in any of this. It is simply set to develop a specific boost pressure. If you have set it too high, there will be fuel cuts and you will turn down the pressure setting on the MBC until there are no fuel cuts.
The boost enrichment does not cool the turbo, but prevents AF mixture dentonation and promotes a well behaved flame front. A forced induction engine also has its ignition timing retarded. Peak pressure occur well into the stroke instead of near TDC. This reduces temperatures and stress. This is not as efficient as normal operation, but so little time is spent on boost that overall mpg's are still good. NA engines also go rich at wide open throttle and will retard ignition if their knock sensors are stimulated. A 4 cyliner turbo engine will be a more efficient than many large displacement NA engines of similar output. The turbo engine is also lighter which helps with mpg's and accelleration. The NA engine does have a persistant advantage of a higher compression ration. All turbo engines have reduced compression ratios to avoid detonation while on boost. Now you can see the motivation behind Saab's variable compression ratio engines.
When a MBC is fitted, the ECU cannot tweak the boost pressure to control knocking, but timing and enrichment changes are all that the ECU requires to manage combustion. A year and 1/2 ago, folks were going nuts about this issue. Their predictions, or wishes, that my engine would self destruct were not realized. But still, it is prudent to maintain clean injectors, plugs and combustion chambers. A high perforance turbo engine needs to be well maintained. The ECU will take care of tuning issues for you. Heat is always your enemy. It reduces power and can lead to adverse combustion conditions. So reducing heat pick up and generation allows for better performance and a greater margin of safety. Good fuel is an obvious requirement. Anyone who cranks up pressure and uses low octane fuel is an idot. The T5 ECU would probably cope where other makers systems would fail.
posted by 208.24.17...
No Site Registration is Required to Post - Site Membership is optional (Member Features List), but helps to keep the site online
for all Saabers. If the site helps you, please consider helping the site by becoming a member.
 |
 |
 |
 |
 |
